JWST Constraints on Primordial Magnetic Fields

TL;DR

This paper uses JWST and Planck data to set stringent upper limits on primordial magnetic fields by analyzing their effects on early galaxy formation and reionisation history.

Contribution

It demonstrates that early galaxy observations tightly constrain primordial magnetic fields, establishing galaxy formation as a sensitive probe for these fields in Gaussian, non-helical models.

Findings

Strong PMFs cause a double reionisation at z ≈ 24 incompatible with CMB data.

Upper bounds on magnetic field strength are < 0.27 nG for n_B = -2 and < 0.18 nG for n_B = 2.

Early galaxy observables are among the most sensitive probes of primordial magnetic fields.

Abstract

Primordial magnetic fields (PMFs) enhance small-scale structure formation through the Lorentz force acting on baryons, boosting the abundance of low-mass halos and their hosted galaxies. We show that the reionisation history calibrated with the UV luminosity function (UVLF) provides stringent bounds: strong PMFs induce a characteristic double reionisation at $z \approx 24$ that is incompatible with CMB measurements of the optical depth, yielding $\sqrt{\left\langle B^2 \right\rangle} < 0.27\,{\rm nG}$ and $< 0.18\,{\rm nG}$ for $n_B = -2$ and $n_B = 2$ respectively at $95\%\,{\rm CL}$ using Planck priors on $\tau$. This establishes early galaxy observables as among the most sensitive probes of PMFs in Gaussian, non-helical scenarios.